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GroupBy samples for What's New in EF Core 6.0 (#3436)
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310 changes: 305 additions & 5 deletions entity-framework/core/what-is-new/ef-core-6.0/whatsnew.md
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title: What's New in EF Core 6.0
description: Overview of new features in EF Core 6.0
author: ajcvickers
ms.date: 09/14/2021
ms.date: 09/18/2021
uid: core/what-is-new/ef-core-6.0/whatsnew
---

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## Improved GroupBy support

GitHub Issue: [#12088](https://github.com/dotnet/efcore/issues/12088).
GitHub Issue: [#13805](https://github.com/dotnet/efcore/issues/13805).
GitHub Issue: [#22609](https://github.com/dotnet/efcore/issues/22609).
GitHub Issues: [#12088](https://github.com/dotnet/efcore/issues/12088), [#13805](https://github.com/dotnet/efcore/issues/13805), and [#22609](https://github.com/dotnet/efcore/issues/22609).

// Coming soon...
EF Core 6.0 contains better support for `GroupBy` queries. Specifically, EF Core now:

* Translate GroupBy followed by `FirstOrDefault` (or similar) over a group
* Supports selecting the top N results from a group
* Expands navigations after the `GroupBy` operator has been applied

The following are example queries from customer reports and their translation on SQL Server.

**Example 1:**

<!--
var people = context.People
.Include(e => e.Shoes)
.GroupBy(e => e.FirstName)
.Select(
g => g.OrderBy(e => e.FirstName)
.ThenBy(e => e.LastName)
.FirstOrDefault())
.ToList();
-->
[!code-csharp[GroupBy1](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=GroupBy1)]

```sql
SELECT [t0].[Id], [t0].[Age], [t0].[FirstName], [t0].[LastName], [t0].[MiddleInitial], [t].[FirstName], [s].[Id], [s].[Age], [s].[PersonId], [s].[Style]
FROM (
SELECT [p].[FirstName]
FROM [People] AS [p]
GROUP BY [p].[FirstName]
) AS [t]
LEFT JOIN (
SELECT [t1].[Id], [t1].[Age], [t1].[FirstName], [t1].[LastName], [t1].[MiddleInitial]
FROM (
SELECT [p0].[Id], [p0].[Age], [p0].[FirstName], [p0].[LastName], [p0].[MiddleInitial], ROW_NUMBER() OVER(PARTITION BY [p0].[FirstName] ORDER BY [p0].[FirstName], [p0].[LastName]) AS [row]
FROM [People] AS [p0]
) AS [t1]
WHERE [t1].[row] <= 1
) AS [t0] ON [t].[FirstName] = [t0].[FirstName]
LEFT JOIN [Shoes] AS [s] ON [t0].[Id] = [s].[PersonId]
ORDER BY [t].[FirstName], [t0].[FirstName]
```

**Example 2:**

<!--
var group = context.People
.Select(
p => new
{
p.FirstName,
FullName = p.FirstName + " " + p.MiddleInitial + " " + p.LastName
})
.GroupBy(p => p.FirstName)
.Select(g => g.First())
.First();
-->
[!code-csharp[GroupBy2](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=GroupBy2)]

```sql
SELECT [t0].[FirstName], [t0].[FullName], [t0].[c]
FROM (
SELECT TOP(1) [p].[FirstName]
FROM [People] AS [p]
GROUP BY [p].[FirstName]
) AS [t]
LEFT JOIN (
SELECT [t1].[FirstName], [t1].[FullName], [t1].[c]
FROM (
SELECT [p0].[FirstName], (((COALESCE([p0].[FirstName], N'') + N' ') + COALESCE([p0].[MiddleInitial], N'')) + N' ') + COALESCE([p0].[LastName], N'') AS [FullName], 1 AS [c], ROW_NUMBER() OVER(PARTITION BY [p0].[FirstName] ORDER BY [p0].[FirstName]) AS [row]
FROM [People] AS [p0]
) AS [t1]
WHERE [t1].[row] <= 1
) AS [t0] ON [t].[FirstName] = [t0].[FirstName]
```

**Example 3:**

<!--
var people = context.People
.Where(e => e.MiddleInitial == "Q" && e.Age == 20)
.GroupBy(e => e.LastName)
.Select(g => g.First().LastName)
.OrderBy(e => e.Length)
.ToList();
-->
[!code-csharp[GroupBy3](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=GroupBy3)]

```sql
SELECT (
SELECT TOP(1) [p1].[LastName]
FROM [People] AS [p1]
WHERE (([p1].[MiddleInitial] = N'Q') AND ([p1].[Age] = 20)) AND (([p].[LastName] = [p1].[LastName]) OR ([p].[LastName] IS NULL AND [p1].[LastName] IS NULL)))
FROM [People] AS [p]
WHERE ([p].[MiddleInitial] = N'Q') AND ([p].[Age] = 20)
GROUP BY [p].[LastName]
ORDER BY CAST(LEN((
SELECT TOP(1) [p1].[LastName]
FROM [People] AS [p1]
WHERE (([p1].[MiddleInitial] = N'Q') AND ([p1].[Age] = 20)) AND (([p].[LastName] = [p1].[LastName]) OR ([p].[LastName] IS NULL AND [p1].[LastName] IS NULL)))) AS int)
```

**Example 4:**

<!--
var results = (from person in context.People
join shoes in context.Shoes on person.Age equals shoes.Age
group shoes by shoes.Style
into people
select new
{
people.Key,
Style = people.Select(p => p.Style).FirstOrDefault(),
Count = people.Count()
})
.ToList();
-->
[!code-csharp[GroupBy4](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=GroupBy4)]

```sql
SELECT [s].[Style] AS [Key], (
SELECT TOP(1) [s0].[Style]
FROM [People] AS [p0]
INNER JOIN [Shoes] AS [s0] ON [p0].[Age] = [s0].[Age]
WHERE ([s].[Style] = [s0].[Style]) OR ([s].[Style] IS NULL AND [s0].[Style] IS NULL)) AS [Style], COUNT(*) AS [Count]
FROM [People] AS [p]
INNER JOIN [Shoes] AS [s] ON [p].[Age] = [s].[Age]
GROUP BY [s].[Style]
```

**Example 5:**

<!--
var results = context.People
.GroupBy(e => e.FirstName)
.Select(g => g.First().LastName)
.OrderBy(e => e)
.ToList();
-->
[!code-csharp[GroupBy5](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=GroupBy5)]

```sql
SELECT (
SELECT TOP(1) [p1].[LastName]
FROM [People] AS [p1]
WHERE ([p].[FirstName] = [p1].[FirstName]) OR ([p].[FirstName] IS NULL AND [p1].[FirstName] IS NULL))
FROM [People] AS [p]
GROUP BY [p].[FirstName]
ORDER BY (
SELECT TOP(1) [p1].[LastName]
FROM [People] AS [p1]
WHERE ([p].[FirstName] = [p1].[FirstName]) OR ([p].[FirstName] IS NULL AND [p1].[FirstName] IS NULL))
```

**Example 6:**

<!--
var results = context.People.Where(e => e.Age == 20)
.GroupBy(e => e.Id)
.Select(g => g.First().MiddleInitial)
.OrderBy(e => e)
.ToList();
-->
[!code-csharp[GroupBy6](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=GroupBy6)]

```sql
SELECT (
SELECT TOP(1) [p1].[MiddleInitial]
FROM [People] AS [p1]
WHERE ([p1].[Age] = 20) AND ([p].[Id] = [p1].[Id]))
FROM [People] AS [p]
WHERE [p].[Age] = 20
GROUP BY [p].[Id]
ORDER BY (
SELECT TOP(1) [p1].[MiddleInitial]
FROM [People] AS [p1]
WHERE ([p1].[Age] = 20) AND ([p].[Id] = [p1].[Id]))
```

**Example 7:**

<!--
var size = 11;
var results
= context.People
.Where(
p => p.Feet.Size == size
&& p.MiddleInitial != null
&& p.Feet.Id != 1)
.GroupBy(
p => new
{
p.Feet.Size,
p.Feet.Person.LastName
})
.Select(
g => new
{
g.Key.LastName,
g.Key.Size,
Min = g.Min(p => p.Feet.Size),
})
.ToList();
-->
[!code-csharp[GroupBy7](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=GroupBy7)]

```sql
Executed DbCommand (12ms) [Parameters=[@__size_0='11'], CommandType='Text', CommandTimeout='30']
SELECT [p0].[LastName], [f].[Size], MIN([f0].[Size]) AS [Min]
FROM [People] AS [p]
LEFT JOIN [Feet] AS [f] ON [p].[Id] = [f].[Id]
LEFT JOIN [People] AS [p0] ON [f].[Id] = [p0].[Id]
LEFT JOIN [Feet] AS [f0] ON [p].[Id] = [f0].[Id]
WHERE (([f].[Size] = @__size_0) AND [p].[MiddleInitial] IS NOT NULL) AND (([f].[Id] <> 1) OR [f].[Id] IS NULL)
GROUP BY [f].[Size], [p0].[LastName]
```

**Example 8:**

<!--
var result = context.People
.Include(x => x.Shoes)
.Include(x => x.Feet)
.GroupBy(
x => new
{
x.Feet.Id,
x.Feet.Size
})
.Select(
x => new
{
Key = x.Key.Id + x.Key.Size,
Count = x.Count(),
Sum = x.Sum(el => el.Id),
SumOver60 = x.Sum(el => el.Id) / (decimal)60,
TotalCallOutCharges = x.Sum(el => el.Feet.Size == 11 ? 1 : 0)
})
.Count();
-->
[!code-csharp[GroupBy8](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=GroupBy8)]

```sql
SELECT COUNT(*)
FROM (
SELECT [f].[Id], [f].[Size]
FROM [People] AS [p]
LEFT JOIN [Feet] AS [f] ON [p].[Id] = [f].[Id]
GROUP BY [f].[Id], [f].[Size]
) AS [t]
```

**Example 9:**

<!--
var results = context.People
.GroupBy(n => n.FirstName)
.Select(g => new
{
Feet = g.Key,
Total = g.Sum(n => n.Feet.Size)
})
.ToList();
-->
[!code-csharp[GroupBy9](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=GroupBy9)]

```sql
SELECT [p].[FirstName] AS [Feet], COALESCE(SUM([f].[Size]), 0) AS [Total]
FROM [People] AS [p]
LEFT JOIN [Feet] AS [f] ON [p].[Id] = [f].[Id]
GROUP BY [p].[FirstName]
```

**Model**

The entity types used for these examples are:

<!--
public class Person
{
public int Id { get; set; }
public int Age { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string MiddleInitial { get; set; }
public Feet Feet { get; set; }
public ICollection<Shoes> Shoes { get; } = new List<Shoes>();
}
public class Shoes
{
public int Id { get; set; }
public int Age { get; set; }
public string Style { get; set; }
public Person Person { get; set; }
}
public class Feet
{
public int Id { get; set; }
public int Size { get; set; }
public Person Person { get; set; }
}
-->
[!code-csharp[Model](../../../../samples/core/Miscellaneous/NewInEFCore6/GroupBySample.cs?name=Model)]

## Preserve synchronization context in SaveChangesAsync

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